Borreliae undergo antigenic variation during the course of relapsing fever infection. Several serotypes are found among the progeny of a single borrelia inoculated into an experimental animal. We are studying the mechanism of antigenic shifts in these spirochetes. Our analysis of Borrelia hermsii lysates with sodium dodecyl sulfate-polycrylamide gel electrophoresis has revealed a major protein that varies in apparent molecular weight between serotypes. Using both polyclonal antisera and monoclonal antibodies, we have shown that these variable proteins are also serotype-specific antigens. We are purifying the variable proteins for further biochemical characterization and employing recombinant DNA techniques to identify the gene or genes involved in antigenic variation.